Many applications, such as robotics, biometrics, automobile security and navigation, medical imaging, surveillance, and others, require a three-dimensional (“3D”) representation of the environment to avoid time-consuming processing steps required to ensure high quality of the specific task. Thus, real-time 3D-imaging has become an important challenge in the design of modern electronic image sensors. Optical systems can provide various advantages such as very fast 3D-data acquisition.
One type of device that has been used for photo-detection in the charge domain is a PPD. A pinned photodiode generally has two implants in the substrate, the doping concentrations of which are chosen in such a way that a fully depleted area is created beneath a very shallow non-depleted layer at the substrate surface. As shown, for example, in FIG. 1, the two implant steps include a deep n− implant 14 and a shallow p+ implant 16 (assuming a p-type substrate 12), where the p+ extends laterally beyond the n− layer in order to create an electrical connection to the substrate 12. At one side of the structure, a poly-silicon gate 18 is provided to enable the transfer of charges out of the PPD region 14 to a sense node diffusion 20. The pinned region 14 defines the photo-sensitive area 24 where photons are converted into electric charges. As long as the transfer gate 18 is set to low potential, the photo-generated charges are stored within the PPD region 14.
Electronic imaging sensors sometimes have an array of (m×n) photo-sensitive pixels, with x>1 rows and y>1 columns. Each pixel of the array can individually be addressed by dedicated readout circuitry for column-wise and row-wise selection. A block for signal post-processing may be integrated on the sensor. Demodulation pixels are a particular class of pixels that are useful for 3D imaging and other applications.
Background light (e.g., infrared radiation), however, can have a significant, adverse impact on imaging devices based on PPDs. For example, many 3D-camera systems operate in the non-visible, near-infrared region of the spectrum (e.g., 780 nm-900 nm). In this range of optical wavelengths, the maximal solar irradiance can amount, for example, to 1000 W/m2/μm, which can result, in some cases, in millions of electrons being generated in addition to the signal-generated electrons. Thus, sensor-level background suppression techniques are generally needed.